Macromolecules are transported across endothelium lining continuous capillaries by intracellular vesicles which shuttle materials between luminal and abluminal membranes. This mode of capillary transport may play an important regulatory role in the movement of substances between the blood and tissue fluids. Studies on the mechanism and regulation of vesicular transport are complicated by the resistance of in situ microvascular systems to precise definition. Micropinocytosis (the initial event in vesicular transport) can be quantitated by measurement of the rates of ingestion of a rhodamine-ferritin conjugate (RhF) into isolated capillaries in suspension. This system is amenable to precise definition and experimental manipulation. In addition, vesicular ingestion can be studied without complication from parallel (inter-endothelial) modes of transport. Labeled molecules of various molecular dimensions and affinity for endothelial surfaces will be employed in addition to RhF to investigate the effects of these parameters on the kinetics of ingestion. Endothelial cell regulation of micropinocytosis will be investigated by determining the effect of metabolic inhibitors and inhibitors of protein synthesis on rates of ingestion. The role of enzymes associated with endothelial membranes will be probed by determining the effect of specific enzyme inhibitors on micropinocytosis. Environmental control of micropinocytic transport will be investigated by determining the effects of various physical parameters, circulatory hormones and vasoactive substances on rates of micropinocytosis.